The present invention concerns a pipette comprising a cylinder which constitutes a cylinder volume with liquid passage; a plunger placed to be reciprocatingly movable in said cylinder volume; and a plunger-operating means for moving the plunger within the cylinder; said operating means comprising a body, a threaded rod, an operating member in cooperation by means of a thread with said threaded rod, and a power means for rotating the threaded rod and the operating member relative to each other; said body being connectable to the cylinder and the threaded rod/operating member combination, to the plunger, for moving the plunger by mediation of the threaded rod and the operating member when the threaded rod and the operating member are rotated relative to each other with the aid of the power means.
The filling of a pipette is based on producing a vacuum in the cylinder volume of the pipette by moving the plunger in the cylinder volume. At the same time, liquid which one wants to fill is conducted into the cylinder volume through the tip of the pipette. The true filling volume of the pipette, i.e., the volume of the liquid quantity that has entered the pipette, depends on quite a number of factors, e.g. the displacement volume of the plunger, ambient air pressure, which in turn is due to temperature and gravity, height of the liquid column inside the pipette, air space inside the pipette, inclination of the pipette (which affects the liquid column height), resilience of the plunger sealing, lifting of the pipette (which gives rise to a downward inertia force of the liquid column), tip leakage, adhesion forces between tip and liquid, etc. The air volume of the pipette, in particular, exerts a great effect on the pipetting accuracy.
For the reasons mentioned, the liquid volume, the true filling volume in connection with pipetting, differs from the corresponding displacement volume of the plunger by even as much as several per cent. The error is highest in high precision pipettes which are meant to be used in a wide operating range, e.g. from 10 .mu.l to 100 .mu.l or 100 to 1000 .mu.l. The highest error percentages are encountered at the lowest rated volumes, even up to 2 to 3%.
In practice, when the true filling volume of the pipette is considered in relation to the displacement volume of the plunger, at the beginning too little liquid enters the pipette, compared with the displacement volume of the plunger, possibly mainly owing to the air volume of the pipette. As operating the pipette is continued, its true filling volume approaches relatively the theoretical displacement volume of the plunger. Therefore the relative error is highest expressly at the lowest filling volume of the pipette.